In many of our Reformed churches (at least CanRC, anyway), catechism instruction for the children/youth is something that’s only done by the church. Sometimes church catechism classes begin in Grade 7, sometimes in Grade 8. Very rarely is anything done to catechize children who are younger. Moreover, all the emphasis falls on the teaching of the church, as if parents basically have no role in teaching Christian doctrine.
Parents should be catechizing their children as soon as they’re able to understand. That means not only teaching them Bible stories, but also teaching them the basic truths of the Christian faith. There are different ways that this can be done. In our family, we use Starr Meade’s Training Hearts, Teaching Minds. This is a devotional based on the Westminster Shorter Catechism. We’ve been using it for at least six or seven years and highly recommend it.
The other day I came across another resource that might be helpful. The Pasadena United Reformed Church has a “Catechism for Shorties.” It’s designed for pre-schoolers. There are 88 simple questions and answers that give a solid doctrinal foundation. You can find it right here.